KOH-Activated Geopolymer Microspheres Recycle Co(II) with Higher Adsorption Capacity than NaOH-Activated Ones

Mechanistic Understanding of Superior Methylene Blue Adsorption Capacity in a Novel g-C3N4 Modified Amorphous Na-Ca-Mg Silicate Adsorbent: Insights from Multinuclear Solid-State NMR Spectroscopy

Silicate-based adsorbents offer significant advantages over traditional materials, particularly due to their superior thermal and chemical stability, enhanced regenerability, and the ability to endure more rigorous operating conditions. In this study, an amorphous Na-Ca-magnesium silicate adsorbent (SAAM) and its g-C3N4-modified counterpart (gCN-SAAM) were synthesized via alkali activation and a subsequent thermal process, respectively. The g-C3N4 modification resulted in a novel hybrid adsorbent with a remarkable methylene blue (MB) adsorption capacity of 420 mg g-1, four times higher than the unmodified sample, setting a new benchmark. Solid-state 29Si (MAS and CP/MAS), 1H MAS, and 13C CP/MAS NMR spectroscopy were used to investigate the complex structures of these adsorbents and their interactions with MB. The local structure of SAAM primarily consists of Q3 Si units, with minor Q0 and Q1 Si species, structural water, and Mg-OH sites. Exposure to MB caused an upfield shift in the 29Si CP/MAS spectrum and enhanced resonances in the high-field region, indicating MB interaction with Si sites. 1H MAS NMR spectra revealed significant interactions between water molecules in the geopolymer-like framework of SAAM and MB. The thermal treatment of SAAM with urea to produce gCN-SAAM enhanced the polymerization of Q3 Si species and increased the relative fraction of Q4 Si sites. This treatment also reduced the intensity of some Mg-OH units, showing interaction with g-C3N4. After MB adsorption on gCN-SAAM, NH2 groups of g-C3N4 disappeared, and shifts in the C2N-NHx and C3N sites indicated their involvement in adsorption, while Si sites remained intact. This thermal method creates a sustainable, cost-effective and efficient adsorbent for MB removal from wastewater. Multinuclear NMR spectroscopy provides detailed insights into the adsorbent's complex structure and MB interactions, potentially guiding the design of improved future adsorbents.

In-situ conversion of geopolymer into novel floral magnetic sodalite microspheres for efficient removal of Cd(II) from water

In the present work, a novel, floral-like, magnetic sodalite microsphere (SODM) was synthesized in situ by using fly ash (FA) and metakaolin (MK) as raw materials and was used to remove Cd(II) from water. Its magnetism can solve the problems of adsorbent recovery and possible secondary pollution. During the static adsorption, SODM shows a maximum adsorption capacity of 245.17 mg/g. The adsorption of Cd(II) on the SODM surface is spontaneous, exothermic, and physicochemical adsorption, which was evaluated by thermodynamics, kinetics, and isotherm studies. During dynamic adsorption, SODM shows a maximum adsorption capacity of 342.74 mg/g in the simulated solution prepared by the deionized water, compared to 215.88 mg/g in the simulated solution prepared using Xiangsi Lake water from Guangxi Minzu University. At 0.5 g SODM dosage in the dynamic adsorption, the adsorption capacity could rise to 632.81 mg/g. These results demonstrated the excellent Cd (II) adsorption performance of the SODM. The adsorption of cadmium on the SODM surface includes the synergistic effects of electrostatic attraction, ion exchange, and surface coordination reaction. Besides, the SODM shows good regeneration performance in both the deionized water and Xiangsi Lake water. The present study explores SODM as an adsorbent for the Cd (II) removal from wastewater and unbolts the industrial applicability of the SODM in the field of wastewater purification.

Adsorptive performance and mechanism exploration of l-lysine functionalized celluloses for enhanced removal of Pb(II) from aqueous medium

In this study, two novel biosorbents of l-lysine grafted cellulose (L-PCM, L-TCF) were prepared for Pb(II) removal from aqueous solutions. Various adsorption parameters were surveyed, such as adsorbent dosages, initial concentration of Pb(II), temperature and pH, using adsorption techniques. At normal temperature, less adsorbent can achieve better adsorption capacity (89.71 ± 0.27 mg g^-1 with 0.5 g L^-1 of L-PCM, 16.84 ± 0.02 mg g^-1 with 3.0 g L^-1 of L-TCF). The pH range of application for L-PCM was 4-12 and that of L-TCF was 4-13. The adsorption of Pb(II) by biosorbents went through the boundary layer diffusion stage and void diffusion stage. The adsorption mechanism was chemisorption based on multilayer heterogeneous adsorption. The pseudo-second-order model fitted the adsorption kinetics perfectly. The Freundlich isotherm model adequately described Multimolecular equilibrium relationship between Pb(II) and biosorbents; the predicted maximum adsorption capacities of the two adsorbents were 904.12 and 46.74 mg g^-1, respectively. The results showed that the adsorption mechanism was the electrostatic attraction between Pb(II) and -COOH and the complexation between Pb(II) and -NH₂. This work demonstrated that l-lysine modified cellulose-based biosorbents have great potential in the field of Pb(II) removal from aqueous solutions.

Oleic Acid-Tailored Geopolymer Microspheres with Tunable Porous Structure for Enhanced Removal from Tetracycline in Saline Water

Tetracycline (TC) in the water body poses a huge threat to the ecological environment. There is a great challenge to develop highly efficient, green, low-cost and reusable adsorbents for TC removal from saline water. Herein, metakaolin-based geopolymer microspheres (MM) modified by oleic acid were proposed for the enhanced adsorption of TC from saline water. Experimental and characterization results showed that the introduction of oleic acid into the MM effectively adjusted the specific surface area, pore volume and zeta potential of the MM, thus accelerating the adsorption rate and enhancing the TC adsorption capacity of the MM. The adsorption process fitted well to the pseudo-second-order kinetic and Langmuir isothermal models. The Langmuir adsorption capacity of TC by the optimal MM, namely MM3 (0.3%, oleic acid), reached 645.7 mg·g−1 at 298 K, which was higher than many reported adsorbents. The adsorption process was endothermic and spontaneous. The MM3 had good adsorption performance of TC from saline water and regeneration performance. Moreover, the breakthrough curves of the MM3 in a column system were correlative with the Thomas and Yoon–Nelson models. The adsorption mechanisms of TC by the MM3 involved Van der Waals forces, electrostatic interactions, hydrogen–bonding interactions, and ion exchange.

Highly Efficient Adsorption of Sr2+ and Co2+ Ions by Ambient Prepared Alkali Activated Metakaolin

This study aimed to explore a low cost and sustainable adsorbent to remove Sr²⁺ and Co²⁺ ions, which are major radioactive ions in nuclear wastewater. The material properties of the alkali-activated metakaoline as a function of soaking time at ambient temperature from 1 day to 7 days were examined by XRD, XRF, SEM, and solid-state NMR. Adsorption isotherms were used to evaluate the appropriate soaking time for the optimal sorption performance for both Sr²⁺ and Co²⁺ ions. The alkali-activated metakaolin soaked for 3 days (BK3) presented the maximum adsorption capacities of 3.81 meq/g (167.5 mg/g) and 4.02 meq/g (118.5 mg/g) for Sr²⁺ and Co²⁺, respectively. The sorption mechanisms for Sr²⁺ and Co²⁺ in the BK3 sample were investigated, and the experimental results indicated that adsorption for Sr²⁺ was achieved via ion exchange. By contrast, surface complexation in combination with ion exchange contributed to the sorption mechanisms for the removal of Co²⁺. Competitive adsorption experiments revealed that the alkali-activated metakaolin favored the adsorption for divalent ions (i.e., Sr²⁺ and Co²⁺), and it was less effective for Cs⁺. Finally, the used adsorbent could be directly mineralized and vitrified by heat treatment to immobilize the Sr²⁺ and Co²⁺ ions.

Ion-Imprinted Polymers: Synthesis, Characterization, and Adsorption of Radionuclides

Growing concern over the hazardous effect of radionuclides on the environment is driving research on mitigation and deposition strategies for radioactive waste management. Currently, there are many techniques used for radionuclides separation from the environment such as ion exchange, solvent extraction, chemical precipitation and adsorption. Adsorbents are the leading area of research and many useful materials are being discovered in this category of radionuclide ion separation. The adsorption technologies lack the ability of selective removal of metal ions from solution. This drawback is eliminated by the use of ion-imprinted polymers, these materials having targeted binding sites for specific ions in the media. In this review article, we present recently published literature about the use of ion-imprinted polymers for the adsorption of 10 important hazardous radionuclides-U, Th, Cs, Sr, Ce, Tc, La, Cr, Ni, Co-found in the nuclear fuel cycle.